Oxidation of semiconductive alloys and products obtained thereby

ABSTRACT

THERE IS PROVIDED A PROCESS FOR PRODUCING FROM CERTAIN SEMICONDUCTIVE ALLOYS, E.G. AN ALLOY OF SILICON AND GERMANIUM, WITH OR WITHOUT DOPANTS, A COMPOSITE BODY HAVING AN OZIDIZED COATING ON A BULK BODY OF THE ALLOY, AND IN WHICH THERE IS AN INCREASED CONCENTRATION OF ONE OF THE ELEMENTS, E.G. GERMANIUM, AT THE INTERFACE AND IN THE SUBSURFACE AND WHEREBY THE OXIDE COMPOSITION IS CONTROLLED AND WHEREIN THE BULK BODY OF THE ALLOY IS SUBJECTED TO A PREDETERMINED NUMBER OF OPERATIONAL CYCLES FROM 1 TO 10 OR MORE, EACH CYCLE INCLUDING OXIDATION, E.G. THERMAL OXIDAION, IN A CHEMICALLY OXIDIZING ENVIRONMENT, E.G. AN OXYGEN-CONTAINING OR YIELDING GASEOUS MEDIUM. FOLLOWING OXIDATION, ETCHING OF THE OXIDE COATING MAY OPTIONALLY BE PERFORMED, OR AN OXIDATION-ETCHING CYCLE REPEATED. NOVEL SEMICONDUCTOR MATERIALS ARE OBTAINED BY THIS PROCESS WHICH AFFORD ADVANTAGES IN FABRICATION AND PERFORMANCE OF DEVICES.

April 3, 1973 A. B. KUPER 3,725,161 OXIDATION OF SEMICONDUCTIVE ALLOYS AND PRODUCTS OBTAINED THEREBY Filed March 3, 1971 6 Sheets-Sheet wa/ swvasouolw 65 55-29593 v n N 2 f E 95 HOLB- 09. w oood 00 3 0 2 INVENTOR ALA/V B. KUPER Apnl 3, 1973 A. B. KUPER I 3,725,151

7 OXIDATION OF SEMICONDUCTIVE ALLOYS AND PRODUCTS OBTAINED THEREBY Filed March 5, 1971 6 Sheets-Sheet 5 SAMPLE a 40 MINUTE OXIDATIONS V) 2 q m U q: 2 Z

l l I l l l I I Z a; I? a '3 m 7,500 5 g 5 5,000 N g 2,500 m J l l l 1 1 1 m 2 3 4 5 e 1 a e OXlDATION-ETCH CYCLES U) U) LL] 2 X 2 R E 0 I0 A A "jg ZZZ At.% Ge in SiGe L N 51-7-20 lk/Vam Mw/m; QM

ATTORNEYS April 3, 1973 A. B. KUPER OXIDATION OF SEMICONDUCTIVE ALLOYS AND PRODUCTS OBTAINED THEREBY 6 Sheets-Sheet 4.

Filed March 3, 1971 SAMPLE C 40 MINUTE OXIDATIONS I 6 7 OXIDATION -ETCH CYCLE S Q\ mszmoomua Aiwmmzxuih L0 INVENTOR ALA/V B. KUPER HOURS ATTORNEYS A ril 3, 1973 A. B. KUPER OXIDATION OF SEMICONDUCTIVE ALLOYS AND PRODUCTS OBTAINED THEREBY Filed March 5, 1971 6 Sheets-Sheet 5 l,oo-

O oxmxmow TIME s s;

- IO MIN. S o 40 17.: E] 60 u I o l I l 0 I0 so M.% Ge in SiGe r A h STEADY-STATE 7 2 u FIRST OXIDATION m I A/ 2 4 I50: o :5 v 2 t INVENTOR og ALANB/(UPER mamas) A TTORNEYS A rll 3, 1973 A. B. KUPER 3,725,151

OXIDATION OF SEMICO IUC" VE ALLOYS AND PRODUCTS OBTA HEREBY Filed March 5, 1971 6 Sheets-Sheet 6 'GB 'RICH LAYER Q o o 3 Si 0R Si Ge 63 o o INVENTOR ALA/V B. KUPE'R @V/MW WW 1; 9' 1 A TTORNEYS United States Patent M 3,725,161 OXIDATION OF SEMICONDUCTIVE ALLOYS AND PRODUCTS OBTAINED THEREBY Alan B. Kuper, 2265 Delaware Drive, Cleveland Heights, Ohio 44106 Filed Mar. 3, 1971, Ser. No. 120,448 Int. Cl. B44d 5/12; H011 11/00 US. Cl. 156-17 14 Claims ABSTRACT OF THE DISCLOSURE There is provided a process for producing from certain semiconductive alloys, e.g. an alloy of silicon and germanium, with or without dopants, a composite body having an oxidized coating on a bulk body of the alloy, and in which there is an increased concentration of one of the elements, e.g. germanium, at the interface and in the subsurface and whereby the oxide composition is controlled and wherein the bulk body of the alloy is subjected to a predetermined number of operational cycles from 1 to or more, each cycle including oxidation, e.g. thermal oxidation, in a chemically oxidizing environment, e.g. an oxygen-containing or yielding gaseous medium. Following oxidation, etching of the oxide coating may optionally be performed, or an oxidation/etching cycle repeated. Novel semiconductor materials are obtained by this process which afford advantages in fabrication and performance of devices.

BACKGROUND OF THE INVENTION The present invention concerns the oxidation of alloys, usually single crystals, of at least two principal component elements and products obtained thereby. Principal among such alloys are those binary systems of silicon and germanium, which will be used for illustrative purposes herein. The principal components of the alloys or compositions of elements in accordance herewith are present in significant amounts, preferably at least about 1.0 atom percent, and are to be distinguished in kind from trace donor or acceptor impurities called dopants such as in the case of Si, which may be doped with other elements, e.g. P, B, Al, Sb or the like, in trace amounts measured in parts per million and usually not exceeding about 500 ppm. As indicated below, dopants may be present in these compositions in the trace amounts usually employed for such materials, but in any event at least two principal component elements are also present.

A principal alloying constituent, such as Ge in SiGe alloy, is to be distinguished fundamentally from a doping impurity not only in that its concentration is significantly larger but that (1) Ge is a group IV element and does not dope Si, (2) Ge is an integral part of the SiGe alloy such that all parameters of the system are affected in a major way by its presence, in contrast to the effect on Si of a dopant, which is principally to change electrical conductivity.

Germanium and silicon are Group IV elements, similar in many respects except in respect to their relative oxidizability, i.e. they are differentially oxidizable. Both are important commerical semiconductors usually used as single crystals. They form a continuous series of solid solutions when crystals are made from a mixture of the two elements. Solid Ge and Si differ in band structure, chemical activity with regard to impurities, and in many other related ways. These differences lead to different technologies for the two, with different advantages in fabrication and use.

In semiconductor technology composite bodies comprising a silicon base, possibly doped, and a silicon oxide coat are currently used. A typical example of such a use is a P-type silicon base having selectively diffused 3,725,161 Patented Apr. 3, 1973 therein a plurality of units comprising each two N-type regions separated from each other and serving as source and drain, a silicon oxide coat spanning the gap between said N-type regions, and an electrode, e.g. of aluminum, applied to the silicon oxide layer. Each such unit serves as a triode and the performance thereof depends on the characteristics of the system. These in turn are dependent on the characteristics of the oxide coat, of the oxidebulk interface, and of the bulk layer below the interface. For example, the offset voltage and the triode transconductance depend principally on the nature of the oxide and interface, respectively. The conductivity between source and drain with the device conducting (and also the energy gap and band structure) depend principally on the composition of the subsurface alloy.

In general, the properties of the oxide coat are important in fabrication and performance of a semiconductor device. Among such properties there may be mentioned the permeability to impurities, built-in charge relative to semiconductor surface, dielectric constant, growthrate and etch-rate, dielectric strength, conductance, optical properties, glass-forming properties, mechanical properties and thermal properties.

It may be stated quite generally that the performance of every semiconductor device comprising a semiconductor body coated with an oxide layer is the resultant of an interplay between the properties of the oxide, the nature of the oxide-bulk interface, and the properties of the bulk layer immediately below said interface. It has now been found that by modifying the properties of these three components in a novel controlled manner, the performance of a semiconductor may be influenced in a desired way.

Where an oxide coat is grown on a substrate consisting of an alloy of elements, the properties of the oxide-bulk interface or of the bulk surface are as a rule different from the properties of the bulk and of the clean, etched surface prior to any oxidation. Where the substrate is a semiconductor, the performance of the body depends to an important degree on the nature of the surface and oxide, the oxide-bulk interface, and also depends on the bulk layer immediately below that surface. Hereinafter the combined properties of the oxide-bulk interface and the bulk layer or subsurface immediately below the interface will be referred to, for short, as the surface properties.

It is an object of the present invention to provide a method for producing semiconductor bodies With controlled surface, interface, subsurface, and oxide properties. This is accomplished by preferential oxidation of at least one of the principal components of a semiconductor alloy. During this process, impurities present in the system in trace amounts will also redistribute (see Atalla 2,953,486), but this is not the subject of this invention.

As is evident from the ensuing description, the present invention utilizes a forced oxidation, e.g. thermal and electrochemical oxidation, as distinguished from the spontaneous oxidation which some elements undergo in air under normal or room conditions, as for example aluminum or silicon. In such cases, the oxide coating forms until it reaches a characteristic thickness, after which further oxidation stops for all practical purposes. With SiGe alloys, spontaneous oxidation at room temperature nearly ceases at approximately 50 angstroms. The oxidation hereinafter disclosed provides much thicker oxidized metal coatings, for example, upwards of 200 angstroms. For most purposes, a will appear from the annexed drawings, coatings having thicknesses upwards from about 800 angstroms to as high as about 13,000 angstroms are produced in accordance herewith and will be found to be the most convenient and thus preferred. At these elevated levels of oxidation, rate limiting is not experienced before the desired redistribution occurs to the significant extents that have been found to enable control of the characteristics of the semiconductor materials or devices formed in accordance herewith. At the low oxide coating thickness obtained with spontaneous oxidation, the oxide breakdown voltages are also too low for practical semiconductor devices.

Brief Statement of the Invention The invention is based on the discovery of a number of surface changes which occur when alloys, as distinct from doped or undoped elemental bulk bodies, especially those alloys of Ge and Si, are oxidized; and on the realization that these changes may be utilized to facilitate processes and to produce products with improved performance as compared to known systems, especially those of Si and SiGe. The said discoveries may be summed up brifiy as follows:

(1) On thermal oxidation of an alloy or composition of elements, e.g. SiGe, the bulk surface (oxide-bulk interface) and a bulk layer immediately thereunder are enriched in the less readily oxidized component, e.g. Ge in an SiGe alloy. The composition of the SiGe bulk layer of subsurface can be accurately adjusted in such a boundary layer.

(2) The oxide grown on the substrate upon a first oxidation consists predominantly of the oxide of the more readily oxidized element, e.g. in an SiGe alloy, SiO with a small Ge content not exceeding as a rule 1 atom percent, and Ge being present partly as elementary Ge and partly oxidized.

(3) By means of a number of operational cycles comprising each etching off a previously formed oxide coat and thermal oxidation, it is possible to arrive at an oxidized element coating which on an SiGe alloy is an SiO coating containing a proportion of Ge exceeding 1 atom percent, again partly as elementary Ge and partly oxidized, and this oxide has properties, such as growthrate, etch-rate, and density, appreciably different from SiO or from oxide first grown on the same sample.

(4) Offset voltage, which is a measure of the oxidebulk system, is less than for SiO on Si.

(5) Using oxidation to adjust the composition of semiconductor subsurface layers by selective oxidation is useful in other binary and ternary systems.

(6) Other oxidants including nitrogen and sulfur as well as other methods of achieving the oxidized state, e.g. electrochemical means, are contemplated for use with such binary and ternary systems, especially SiGe, to form the corresponding oxides, nitrides and sulfides.

Brief Description of the Drawings In the following description reference will be had to the annexed drawings in which:

FIG. 1 is a diagrammatic representation of the volume changes in an oxidation-etching cycle.

FIG. 2 is a graphic representation of X-ray microprobe analysis of an oxidized SiGe alloy compared with an unoxidized control.

FIG. 3 is a graphic representation of the etching rates of SiO produced by a first oxidation on an Si and SiGe substrate.

FIG. 4 is a graphic representation of the MOS (metaloxide-semiconductor) differential capacitance vs. voltage of a system according to the invention.

FIGS. 5 to 8 are graphic representations showing the mass change and accompanying changes in oxide thickness and etch-rate (in all but FIG. 6), on different SiGe alloys and different oxidation times on repeated oxidation-etching cycles.

FIG. 9 is a graphic representation of oxide thickness vs. timefor variety of different SiGe alloys and Si control after a first oxidation and a so-called steadystate oxidation.

FIG. 10 is a graphic representation of the thickness of the oxide layer vs. composition of SiGe alloys after one hour of oxidation for a first oxidation and for a socalled steady-rate oxidation.

FIG. 11 is a graphic representation of the etching rate of the oxide coat vs. composition of SiGe alloys for first and steady-state oxidation.

FIG. 12 is a graphic representation depicting the specific oxide mass (total mass/area) of a first and steady-state oxidation vs. time.

FIG. 13 is a graphic representation showing the Ge pile-up as gradient profiles in the boundary layers at and below the interface and the Ge content in the oxide in a first oxidation and during three phases of a steadystate oxidation as time progresses.

FIG. 14 is a diagrammatic representation of one manner of application of the invention.

Detailed Description of Drawings and Invention In FIG. 1 M signifies the mass of an SiGe alloy prior to oxidation. M signifies the mass of the oxidized body (for simplicity only one surface is shown oxidized; in practice, all surfaces are oxidized unless shielded) and it is seen that the surface of the substrate which now forms the oxide-substrate interface has receded to below the original surface while at the same time the oxide layer has grown to project beyond that surface, the reason being that in the course of oxidation Si and Ge from the surface is converted into oxide. M in FIG. 1 signifies the residual substrate left after etching off of only the oxide layer from M In the following description the sequence of operations leading from M to M i.e. oxidation of an etched substrate and etching off of the formed oxide layer will be referred to as one cycle. In one cycle M M signifies the weight loss in that cycle and M M signifies the weight gained during the oxidation phase of the cycle. There is defined as mass ratio M M M -M All masses are to be taken as specific masses, i.e. mass It has been found in accordance with the present invention that while for oxidation of pure Si, f l, for only a small removal of Ge from an SiGe sample, f 1. This finding enables determination of the content of Ge in oxide coats grown on SiGe substrate. From the determination of the mass ratio for various SiGe alloys and different oxidation times, the following conclusions were drawn:

(a) In an SiGe alloy, Si is oxidized in preference over Ge.

(b) The above preference is approximately independent of the compositions of the alloy and the time of oxidation, and a first oxide layer grown on an SiGe alloy contains Ge in an amount not exceeding 1 atom percent.

The experiments comprising oxidation, etching and weighing which led to the above and following observations were conducted as follows:

(1) Samples.-Two types of SiGe slices, each of 1.5- 2 cm. were used: polycrystals approximately (111) orientation heavily doped, 20-40 atom percent Ge, mainly for oxidation studies and single crystals up to 4.5 atom percent Ge, mainly for electronic studies.

Polycrystals were commercially produced thermoelectric material N and P having a resistance of approximately 0.001 ohm-cm, with large grains and twins (approximately 0.5 mm.), without cracks or voids, which etched and oxidized uniformly. They were solidified by the molten zone technique of Wang and Alexander (F. Rosi, Solid St. Elects, 11, 833 [1968] to obtain uniform ingot composition. In 10-15 atom percent Ge composition, composition varies less than 1% and boron doping,

on the order of 0.5%, along cms. length (RCA 3rd Quarterly Report, SiGe Thermoelectric Materials and Module Dev. Program July 1968-September 1968, ALO (2510)3 A'EC Res. & Dev. Report Cat. UC33, TID 4500). My electron microprobe X-ray fluorescence analysis of N-type salmples indicated that the composition in 5 calibrated to full-scale with the 1 mg. weight. These selected small areas (approximately 100a squares) of settings are checked and then locked. After oxidation, polished slices was uniform to better than 1% and phosthe sample is weighed (M After etching off the oxide, phorus was uniformly distributed. the sample plus the 1 mg. calibrating weight is weighed Single crystals were N and P 0.2-40 ohm-cm. grown 1O (1 mg.M ending the weighing cycle. in a conventional silicon crystal puller geared down to (5) Thickness was measured by conventional interferopull at rates of less than 2 cm./hour to minimize the metric technique using evaporated Al over the etched effects of constitutional super-cooling. Crystals up to 9 oxide-to-sample step. Good agreement was found (see cm in length were grown with shiny surfaces and 6 clearly discussion of FIG. 3) when thickness was also determined defined facets. The bottom of each crystal was polyby SiO interference colors (W. A. Pliskin and E. E. crystal resulting from melt enrichment. X-ray topograph Conrad, IBM J. Res. Develop. 8, 43 [1964]), so color of one slice of 3 atom percent Ge crystal showed regular was used in most cases. An important first experiment annular striations like those encountered in heavily doped was done which showed (Table 2) that the doping Si or Ge (1. A. M. Dikhotf, Phil. Tech. Rev. 25, 195 impurity did not appreciably interfere with the com- [1963]). position effect being studied. In Table 2 an increase in Composition of each slice was determined by density thickness is shown of about 10 A./atom percent Ge for (I. Dismukes, J. Phys. Chem. 68, 3021 [1964] resistivity oxide grown on a fresh surface. by 4-point probe. Data are given in Table l. Sawed samples i were heavily etched and lapped and polished to remove TABLE 2' TOUR'HOUR FIRST OXIDATION damage. Final polish was diamond (0.25-0.5tt) on cloth. It should be noted that an initial etching step on a given Qhmcm, Oxide color 16 i sample may, in practice, he desired. The final step to Silicon 0 01 N Violet 10 600 produce an oxidized composite will, of course, be oxidab3 t 51' 3;1i (}jj 0 g a e' ijjjj 101700 tion. To expose an enriched alloy surface, etching or rc- 5 Percen Y moval of the oxidation product coating will be the final atom percent Ge M021 N HOUO step.

TABLE 1.-SiGe SAMPLES Atom Total Conduc' percent area Thickness tivity Sample Number Crystal Ge (cmfl) (microns) (ohm-cm.) Type A Poly 36.6 2.87 356 0.0021 N B d0 36.5 3.41 183 0.0021 N o ..do 20.7 3.69 243 0. 0017 P D Single 4.0 3.84 257 0.3 P

i (6) Ge Surface pile-up was measured directly by use oXlda-tloll was done In a conventional hoTlZoIltal of the J eolco electron microprobe analyzer measuring the wet Y' system Consisting of a Partlally closed tube Ge Lu X-ray line at 5.5, 10 and 25 kv. in order to peneat AmhieHt-Pressure OXYgeH was bubbled trate to different depths. An unoxidized control was comthfohgh deionized water at 95 a T'lme was measured pared to an oxidized adjacent piece of the same sample. from time of insertion of sample in a small boat to time (7) O id electrical properties, were measured on the of withdrawal with no correction for warm-up for times B t 75C capacitance b id d h Bconton 71 A of 25 minutes or more. Runs of 10 min. were accurately capacitance timed y inserting y the sample for fast P using (8) Oxide infrared transmission was measured on the a long q Vaeuum-ehuek- No efieet on weight was Perkin-Elmer Model 21 double-beam spectrometer in the found when samples were baked for 10 in y Y' 5O vicinity of 9 and 12 microns comparing an oxidized g at end of f sample to a matched sample without oxide.

OXlde Etch was buttered HF g. NH4F The expression first oxidation used hereinbefore and HF 250 water) at agitated a to be used hereinafter refers to the oxidation of a surface Teflon basket. Rate of etching of oxide on Si was found on hi h there are no traces of any Previous appreciable t a 9 m To determine total time to oxidation history. Consequently, fof a first oxidation it is move oXitle, the basket was lifted out of the liquid every always necessary to first etch the surf-ace of the substrate 15 se Time w measured to that at which the Sample down to a depth sufiicient to remove completely any oxide floated when returned to the liquid since the sample is layer d ufi-ici nt layer of the bulk that may have hydrophobic in contrast to the oXlde- To he sure all been affected by previous appreciable oxidation (see, for, oxide was removed, the sample was returned to the bath l di i of FIG- 5 for one additional minute. The etched sample was then The r en of Ge in a first oxide layer grown at 1065 rinsed in deionized water, blotted dry and returned to C on a pclycrystal Si sample f 14 atom percent Ge the weighing chamber, whereby the cycle was eompletedwas determined by infrared absorption measurement, y Weighing, it was verified that the etch bath etched which showed a slight shift in the direction that was to the sample y to a negligible eXteIlt- However, the be expected from Ge. The sample was run together with a Water Was found to etch the sample about 9/Lgt/mihmatched unoxidized SiGe blank. Pure Si was oxidized and so rinsing was p Sputtering and other P y run in a double-beam spectrometer, similarly against a and chemical techniques may also be used to remove tched blank, Comparing the two, peaks shifted from oxidized Product eoatlhgsapproximately 9.25,u (SiO to 9.4,u, and from 12.3;1.

(4) Microbalance weighing was done with a Cahn (S10 12,6

Electrobalance Model 1 using nefull-scale s Together with the preferential oxidation of 51 from an Precision is i as and fepfofhlelbllity t The SiGe alloy there occurs a Ge pile-up at the interface and balance setting shifts so little with time that overnight in the bulk layer immediately below the interface. This runs are possible. Sample is cleaned, balanced and pile-up was determined by an X-ray microprobe surface balance time-stability checked. For maximum sensitivity,

the sample is made to weigh about 15 mg. which is the maximum weight allowed using the full beam of the balance. The sample itself is the pan and lightweight hangers of a few mg. weight are used. Thus with sample mounted, the beam is balanced to zero (M -=0) and analysis of a 36.5 atom percent Ge P-type sample after first oxidation. This sample was scribed and broken into two parts one of which was oxidized to a thickness of 5200 A. and oxide removed. The two pieces were mounted side by side and X-ray fluorescence counting of Ge Lu line was done at several points on both pieces. (It is interesting grown on an 0.2 ohm-cm. N-type SiGe alloy of 2.5 atom percent Ge content. The results of the measurement are shown in FIG. 4 where capacitance is ploted against voltage and it is seen that the first oxide gives a fiat-band voltage of approximately as compared to 4 to -6 v. ob-

that, comparing the two pieces, the unoxidized control tamed with 2200 A. of S10 grown by the same process looked darker. That is, the effect of Ge enrichment on pure Si.

of the surface was visible with the unaided eye.) Counts, The mass changes occurring during several cycles of after small background correction and gmall begmscltilr- 10 minutes oxidation are diagrammatically illustrated in rent normalization, were compared at 5. 10 an 2 v. 10 FIG. 5. Part (a) of FIG. 5 plots M -M and M M The data are shown in Table 3 together with an esti- (see FIG. 1) as a function of the number of oxidation/etch mate of depth that is sampled by the beam. Depth esticycles. In Part (b) of FIG. 5 the lower curve plots the mate was obtained from measurements of depth in Cu oxide color thickness (left-hand side ordinate) as a funcfrom which 95% of X-ray yield came (U. Schmttz, L. tion of the number of cycles, and the upper curve plots Ryder, W. Pitsch, 5th Intl. Cong. X-Ray Optics & Microthe oxide removal time by etching (right-hand side ordianalysis, Tu gf P- Y 1186 ofthe elecnate) as a function of the number of cycles. Cycles la, tron TaPge felatlon and NIXOH, lb, and 1c of FIG. 5 are considered each as first oxida- R y PY, Cambrldge P- tion since in each case after the weight cycle, i.e., after A M was measured, a layer of 4 microns, i.e. 4X10 A. or a= V2 more was etched off. Oxide thickness in this as well as M other tests herein set forth was determined by SiO color XR=aA/pzV2 21nd eizclhGresrnglal Ztlmelkifn sirlne cyclesl. Indegd lit is seer; rom OX1 e ayert lC ness an E p z fi g j i g ig oxide layer removal time for these three cycles are about 55 i; &3 g i 6 i 1 3 a for 25 the same, which confirms the assumption that each of these PF- cycles may be considered as a first oxidation. R From the values in Part (a) of FIG. 5 the mass ratio For the compositions 36.5 atom percent Ge and using the i f each cycle can be calculated y p l g them V dependence of electron range relation, the depth esti- Wlth known fatlOS for an OXlde grown 011 P It y mates of Table 3 are obtained. be concluded that the first oxide grown is nearly pure TABLE 3.-SURFACE Ge L04 X-RAY DATA Average Back- Beam counts ground current Relative Estimated V (10 sec.) (10 sec.) (na.) intensity range (,u)

. 1 7,017 34 1. 64 0. 2 ggg iiiioi 4,557 i 107 i 36 1 19,164 1 44 1.36 0. 7 are :2 1 a .2 25 control 34:287 2,596 i 45 1 i The results of the above measurements are shown E t-hat consequent-1y {be samp 1e surface must be graphically in FIG. 2 in which the relative X-ray yield g s layer is etched ofi and g f fl g g the remaining sample is reoxidized. In this case the oxi- 9 fif d S 3 1 f f 1S i i g 45 dation is no longer a first oxidation. In the then following g a f s l g g 6 i 3; the Ge cycles the same procedure is followed, i.e. the oxide alone concentration approaches gradually that of the Ge content 1; fg f f 33 332 gg 32??? i ig g gfi jg g i z i l a igfi gg fi 5 31?; 2 fi gi concentration FIG. 5 it is seen that in these cycles a profound change is 6 y b t g' (on on an occurs as compared to the first oxidation cycles 1a, 1b SiGie iflloy c dfisizts gf di r ih anti y of sis, it y vas to be i Tkllis is 3 strikiligly H tilin gxide f T e sampe oxi izes rst to go ye ow wit a tone 0 fg gi g 5223 23 552 22:52:25 rose pink) about the sameon all grains. In cycle 2 under t H 2 g h h r th thi kness the same conditions there 18 obtained a IlCh blue and the ig a y as s ownsm d g g cocntaim etch time of the oxide of this cycle almost doubles. In ihg lf t p r ce iii g fig at 10630 c. foryone hour subsequent cycles color and etch ginlile change only slightly. in wet oxygen, is plotted against etching time in minutes. S:;g; :531 2 322223 gij: Initial and final thickness was measured interferometrlcalafter cycle 2 the center of the Sample Surface appears sat f liiiifiiitfiiifif fii as of e dized ai on si de the sample in the same boat and this pear i do follow gran: i i ig 0 control oxide showed a slightly different color. This color 1 22 232 EE 35;? s g f gg difference persisted as sample and control were etched tocycle 4 about 40% iiifii oi iii. tiiilffiiriil ii iitifie iv ir ail 2. 352. The a .Sample Was oxldlzfd and sighed m ggg The color (inference was checked at .t=5() minutesby i g a in fi iii 2231233 d f iieisl y tizliih ed ar id tc li ed surface a t not muc h ox de variat ons in mass 0 sampe consume u gg z g g gi g g gs of refractlon of t e first l variation in oxygen taken up. One month elapsed between An important difference in first oxide grown on SiGe as cycles 6 and 7 as compared to a penod of thg order of 1 compared to Si control was discovered by MOS capaciday between other G tance measurements. Measurements were conducted on a A Sample of an 5165 alloy con'tzllnmg m Percent system comprising an evaporated aluminum counter-elec- Ge p T was OXldlZed and etched 111 Cycles trode applied to an approximately 2200 A. first oxide of 40 minutes oxidation and the results are shown in FIG.

7. Data M -M sample consumed, are like FIG. 6 for cycles 3-7, suggesting variations around a steady value. Cycles 8 and 9 show increased M M in each cycle like FIG. 5. Oscillation in M M data is clearly seen with complementary oscillation in M -M data. This suggests weight loss due to evaporation of Ge is affecting the data. This sample oxide was violet-purple after cycle 1 but mottled purple to jade green after cycle 2. First oxide was like polished Si. Second oxide had little dark regions on the sample surface which were present before and after oxide was removed. Cycle 3 caused the sample to appear more uniformly matte before and after oxide was removed. This uniform matte surface persisted subsequently.

FIG. 8 shows the results of the oxidation of a sample of composition 20.7 atom percent Ge (sample C, Table 1) in cycles of 40 minutes oxidation time. Mass of oxygen taken up per unit area by the sample surface is roughly the same as shown in FIG. 7 (for sample with 36.5 atom percent Ge for 40 minutes), but comparing cycles 3 through 8, mass of sample consumed is somewhat less. This suggests that after first oxidation a monotonic dependence on composition exists.

On the basis of the raw weighing data represented in FIGS. 5 to 8, a distinction is made between a first oxidation and oxidation cycles 3, 4, 5, etc., which has been referred to and will be referred to hereinafter as steady-state oxidation.

In FIG. 9 the thickness of the oxide layer is plotted against the timefor various samples including pure Si and for first (1) and steady-state (S) oxidation. The solid line in FIG. 9 joins points for pure Si. It is seen by the pair of points at 25 minutes that the first oxide of sample D of Table 1-4 atom percent Geis approximately 100 A. thicker than oxide on Si, and first oxide thickness increases about 10 A./atom percent Ge (see Table 2).

In FIG. 10 the thickness of the oxide layer after oxidation for one hour is plotted against the Ge content in the SiGe substrate for first (1) and steady-state (S) oxidation and it is seen that even a low Ge content induces a small extra oxide thickness.

Etch-rate vs. SiGe substrate composition for first and steady-state oxidation is plotted in FIG. 11. At 4 atom percent Ge, etch-rate of the first oxide is approximately that of SiO At 2046.5 atom percent Ge, etch rate of the first oxide is approximately 85%, and of the steady-state oxide 57%, of the SiO rate and independent of sample composition. Buffered HF etch is carried out at approximately 22 C. (IO-minute first oxidation does not seem to be consistent with longer time. This may indicate excess Ge.)

In FIG. 12 the total mass/area of first oxides and steady-state oxides are plot-ted against time for samples A and B of Table 1. The result for the first oxidation (lower curve) agrees closely with published values for growth of oxide on pure Si (solid line) which is surprising. It means that Ge at the growing oxide/SiGe interface does not appreciably impede the growth of SiO Against this, the results for steady-state oxidation differ from those for Si and this is discussed further below.

Some of the mass ratio, from the first oxidation data in FIGS. to 8 are 0.889 as compared to 0.8775 for pure SiO Some deviate from this presumably because of weight loss caused by evaporation of GcO It is, however, important to realize that the total mass of oxide, M2,-"M3, is measured (FIG. 12) independently of weight loss from the system and first oxide agrees closely with SiO data.

By referring once again to FIGS. 5 to 8 it is sen that the weighing data after first cycle shows lower observed oxygen up-take (Mg-M1) than first oxidation and a much larger sample consumption (My-M Large variations in mass are evident but average values may be considered for the purpose of analysis.

Steady-state oxide grows thicker than first oxide in one hour. As shown in FIG. 10, the thickness of oxide in one hour in the limit of a few percent Ge in the bulk is about 6000 A., about 11% greater than on pure Si, and it increases about 20 A./atom percent Ge. That is, most of the extra thickness of steady-state oxide is found even at low concentrations. The rate is relatively insensitive to more Ge thereafter, much like first oxide. However, after 16 hours, oxide thickness is close to that expected for SiO on Si.

Etch-rate is markedly lower on steady-state oxide (FIG. 11) grown 10 minutes to 1 hour, dropping to about 57% of SiO rate, independent of bulk composition between 20- 40 atom percent Ge. However, for thick oxide, grown 16 hours, etch rate is like Si0 rate.

Thickness by color is in good agreement with stepheight measurements, i.e. index of refraction is not sensitive to oxide composition.

Total specific mass (M M of steady-state oxide vs. time of FIG. 12 is seen to be more massive when compared to specific mass of first oxide at one hour. The excess mass appears ot be decreasing with time, being less large at four hours. Since, after 16 hours oxidation compared to Si control, oxide thickness and oxide etch rate are nearly the same, it is concluded that excess mass is a small percent of total oxide mass after 16 hours. By comparing excess mass from the upper curve of FIG. 12 and excess thickness from 'FIG. 9 after one hour, density of oxide relative to SiO can be obtained.

Thus mass, thickness and etch rate suggest that initial excess of substrate Ge is incorporated into the growing oxide. Presumably this occurs because of piled up Ge which remains after first oxidation. After about two cycles an approximate steady-state is reached in which Ge is incorporated during the first time of oxidation. This effect is diluted out by continued long-time oxidation which then becomes much like first oxidation. Since the effect occurs early, it is sensitive to initial surface conditions before each oxidation. This accounts for the large variation in sample up-take. It should be noted that oxygen up-take does not show large variation (FIG. 6) which indicates that Ge is incorporated into the oxide without oxygen.

The distinction between a first oxidation and the steadystate oxidation is diagrammatically represented in FIG. 13. 'Part (a) of FIG. 13 represents a first oxidation while Parts (b), (c) and (d) of FIG. 13 represent three phases during a steady-state oxidation, Part (b) approximately 1 minute, Part (c) approximately after 40 minutes, and Part (d) approximately after 4 hours. Solid curves are Ge concentration plotted against distance in oxide layer and in SiGe substrate. Vertical solid lines mark the gas/oxide and oxide/substrate interfaces in each of FIGS. 13b, 13c and 13d as in 13a.

Referring to Part (a) it is seen that in the SiGe bulk there is a pile-up or increase in concentration of Ge toward the interface which reaches its maximum value at the interface. In the oxide layer there is a small Ge content of the order of 1 atom percent which drops along a concentration gradient profile toward the oxide surface. It is considered that the Ge in the oxide layer is pre dominantly in the form of elementary Ge and not oxidized.

At the initial phase of steady-state oxidation, depicted in Part (b) of FIG. 13, an initial transient occurs because of Ge surface enrichment resulting from previous oxidation. During this transient, excess Ge is incorporated in the growing oxide, some Geo evaporates, and other transient effects may occur resulting in pitting. As the oxide layer grows in thickness (Parts (c) and (d)), the Ge content and Ge distribution in surface and oxide changes and approaches the same gradient profile as in a first oxidation.

Based on all the above observations and conclusions, the invention in its broadest aspects consists in a process for producing from a semicond-uctive alloy, e.g. an SiGe alloy, a composite body having a controlled coating comprising mainly the oxidation product of the more easily oxidized element of the alloy and a controlled pile-up at the oxide-bulk interface of the element less readily oxidized in the boundary layer, wherein the substrate is subjected to an oxidizing atmosphere. Preferably, the exposure is to an oxygen-containing atmosphere at an elevated temperature within the range of 700-1300 C. The oxidation step may be preceded by or the process terminated by an etching step to remove oxidized element from the surface of the composite body. The invention also is in the products produced by the process and in its use as a fabrication technique or as an intermediate step in semiconductor processing.

In accordance with one embodiment of the invention, the substrate is subjected to one single etch/oxidation cycle, thereby producing a composite body with an oxide coat containing Ge in an amount not exceeding 1 atom percent.

In accordance with another embodiment of the invention, the substrate is subjected to more than one of said cycles to produce an oxide coat containing an increased proportion of Ge, usually more than 1 atom percent Ge, the etching after each oxidation being so designed as to remove essentially only the oxide coat formed in the previous cycle except in the last step in which the oxide coat may be retained. The number of cycles may be from 1 to 10 or more, if desired. The number of cycles, the temperature, and other parameters of oxidation, e.g. time, will be determined by the final concentration of Ge desired in the oxide coating or at the surface of the bulk material. After a number of cycles for a given alloy, further oxidation/etch cycles do not confer such a material change in Ge concentration that further cycling is beneficial.

If desired, it is possible in accordance with the invention to etch off the oxide coat produced by oxidation after the last cycle to produce a product with no oxide coat and having a predetermined G pile-up at the surface; such a product can be used for further processing in any desired way.

While the invention has been exemplified with a specific class of alloys and oxygen or water vapor as the oxidant material, it will be understood that the principles of this invention may be applied to other binary and ternary alloy bulk bodies. Instead of oxygen, other oxidants for the differentially oxidizable alloy moieties may be used in partial or total replacement for the oxygen, e.g. sulfur, nitrogen, etc., to form the sulfides or nitrides as a coating on the bulk material. Other oxidation processes, such as anodic oxidation, may be employed.

By means of the method according to the invention,

it is possible to achieve, among others, the following advantages in the semiconductor field:

(1) Improved MOST (metal oxide semiconductor transistor) devices because of lower or controllable offset voltage, higher lateral surface mobility (at high Ge concentration), higher transconductance, and better oxide;

(2) Improved bipolar devices, for example, lower nonideal surface current and reduced base resistance;

(3) New-type resistors, capacitors and other passive elements, e.g. improved MOS capacitors by control of fiat-band voltage, conductance, specific capacitance;

(4) More reliable devices whose Ge-containing oxide coat is more dense and possibly less permeable than pure SiO (5) New metal semiconductors or Schottky-barrier devices;

(6) SiGe heterojunction devices fabricated with advantages in photo-absorption and as wide-narrow gap diode, as for example, wide-gap emitter;

(7) Lateral transistors improved by more favorable lateral injection into Ge-rich surface base-layer;

(8) Improved photodevices taking advantage of differing energy-gap in the subsurface relative to the bulk body.

One embodiment of a semiconductor product produced in accordance with the invention is shown diagrammatically and by way of example only in FIG. 14. In that figure a substrate 1 is shown consisting of an Si or SiGe base on Which a thin layer of an SiGe alloy is grown epitaxially, which layer constitutes the substrate for oxidation in accordance with the invention. Discrete circular Zones 2 are defined by adequately masking the substrate, and oxidized. The structure of each oxidized zone 2 is shown in the upper part of FIG. 14. It comprises a Gerich layer 3 in which there is a Ge pile-up, e.g. as in FIG. 13, and an oxide coat 4 of a desired Ge content, depending on whether the coat was produced by oneor multi-cycle oxidation. The substrate below layer 3 is not shown in the upper part of FIG. 14; it is the same as prior to oxidation.

Other devices and integrated circuit fabrication proc esses which are contemplated by this invention are:

(a) Selective use of alloy and oxide for selective oxidation, selective etch-down to make configurations, selective masking of some regions of substrate Si relative to other regions, controlled by variations in oxide properties,

(b) Control of properties over a semiconductor surface by pre-depositing alloy and oxidizing as described above,

(c) Inserting Ge-rich regions by this process to utilize properties such as different mobility, energy gap, ionization coefficients, thermal conductivity, stress-moduli, potential barrier-heights in metal-semiconductor configurations, dielectric constant,

(d) Method of fabricating Ge-Si or near Ge-Si heterojunction,

(e) Use of layers from the process to advantage in ion implantation,

(f) Take advantage of different properties of impurities in Ge-rich region resulting from the process, relative to other regions; different properties being for example diffusion rates, solubilities, segregation, and energy levels of impurities, wetting and alloying, and contacting properties,

(g) Use surface layer or oxidation as an intermediate step in fabrication, not necessarily retained in completed device,

(h) Control of surface potential and/or interface-state densities in oxidized devices and integrated circuits,

(i) Use of multiple layers by combining epitaxial deposition of alloy with this process, for example to make a sandwich of SiO /Ge/Si,

(j) Oxidize SiGe on Ge in order to form SiO thermally on Ge,

(k) Use deposited (not thermally grown) SiO- in regions where Ge pile-up is not desired in this process,

(1) Use anneal post-treatment to advantage.

What is claimed is:

1. A process for producing a composite body from a bulk body of a silicon/ germanium alloy containing more than 1 atom percent germanium, said body being characterized by (a) a coating comprising silicon dioxide, (b) an interface between the silicon dioxide-containing coating and the bulk body and (c) a boundary layer contiguous to said interface having an increased concentration of germanium therein; comprising the step of oxidizing the bulk body with an oxidant for silicon.

2. A process in accordance with claim 1 wherein the silicon dioxide coating has a thickness greater than 200 A.

3. A process in accordance with claim ll wherein the silicon dioxide coating has a thickness greater than 800 A.

4. A process in accordance with claim 1 wherein the silicon dioxide coating has a thickness between 800 and 13,000 A.

5. A process in accordance with claim 1 in which the temperature range for the oxidation step is from 700 C. to 1300 C.

6. A process in accordance with claim 5 in which the silicon/germanium alloy is first epitaxially deposited on a silicon base.

7. A process in accordance with claim 1 which is additionally characterized by the step of etching the surface with an acid reactive only with the silicon dioxide coating alternately performed with said step of contacting with an oxidizing agent.

8. A process in accordance with claim 7 in which the steps of removing the silicon dioxide coating and oxidizing the bulk surface are repeated until steady-state oxidation is achieved.

9. A process in accordance with claim 1 wherein the process consists essentially of the steps of (a) etching the surface with an acid reactive with SiO and (b) oxidizing the etched surface with an oxygen-containing gas at a temperature in the range of from 700 C. to 1300 C. to yield an Si0 coating thereon, the boundary layer being enriched in germanium.

10. A process in accordance with claim 7 in which the semiconductive alloy consists of from about 4 atom percent to 37.5 atom percent of germanium, balance silicon, the oxidant is oxygen, and the reactive acid is a dilute or buffered aqueous solution of hydrogen fluoride.

11. A process in accordance with claim 1 in which the terminal step is etching to remove all traces of SiO 12. A process for producing a composite body from a bulk body of a silicon/germanium alloy containing 1 14 atom percent germanium, said body being characterized by a boundary layer in which the concentration of the germanium is in excess of the concentration of the germanium in the bulk body comprising the steps, alternately performed, of (a) contacting the bulk body with an oxidant for the silicon at a temperature in the range of from 700 C. to 1300 C., to form a coating of the oxidation product on said bulk body, and (b) removing the oxidation product coating from the surface with an acid reactive only with the oxidation product coating.

13. A process in accordance with claim 12 in which the final step is oxidation.

14. A process in accordance with claim 12 in which the final step is removing the silicon dioxide coating.

References Cited UNITED STATES PATENTS 3,287,162 11/1966 Chu 317--235 AG 3,441,812 4/ 1969 Bucs et al. 317-235 AG 3,398,029 8/1968 Yasufuku et al. 317-235 AG 3,342,567 9/1967 Dingwall 317-235 AP ALFRED L. LEAVITT, Primary Examiner M. F. ESPOSITO, Assistant Examiner US. Cl. X.R.

11720l, 118; 75-134 G, 134 S; 317-235 AG, 235 AP 

